This invention relates to a method of and apparatus for monitoring a level of a grinding charge in a grinding mill. The invention is particularly concerned with autogenous and semi-autogenous mills and finds application in run of mine milling processes employed on gold and platinum mines.
In run of mine milling it is necessary to maintain the feed rate of uncrushed ore into a mill at an optimum level in order to produce the desired fineness in the end product.
If the feed rate is too high the mill overloads and if the feed rate is too low the mill becomes underloaded. In both cases the mill efficiency deteriorates rapidly.
A mill of the kind referred to includes a cylinder which is rotated by means of a motor. The mill load within the cylinder is caused to rotate and cascades onto an impact point inside the cylinder. The position of the impact point is related to the level of the charge and a microphone has been used in the past to establish the location of the impact point. The microphone detects the sound level caused mainly by the impacting load and as the sound level varies when the location of the impact point changes an operator is able, through experience, to alter the feed rate of the ore into the mill accordingly. Thus as the loading of the mill is increased the point of impact rises and conversely if the charge level drops so does the impact point. Clearly if use is made of a microphone to detect the sound level at the point of impact then the microphone will provide an indication of optimum operating conditions. However if there is a reduction in sound level then the microphone is not able to indicate whether the rate of feed of ore should be increased or reduced.
Sound-based systems of this kind are described for example in the specifications of U.S. Pat. Nos. 2,766,941 and 2,235,928. The specifications of UK Pat. No. 1105974 and U.S. Pat. No. 3314614 relate to the use of separate microphones for separate compartments in a multi-chamber mill, while the specification of U.S. Pat. No. 2,833,482 discloses the use of a first microphone at the "solids" end of the mill and a separate microphone at the "water" end of the mill.
U.S. Pat. No. 2,405,059 is concerned with a mill control system which makes use of multiple sensors which are in physical contact with the rotating mill shell. The objective is to eliminate errors which are present in devices which are responsive to air-borne vibrations. The sensors are symmetrically positioned around the shell to give "average values of grinding performance".
Russian Pat. No. 869 809 shows a sonic method of diagnosis of the state of a ball mill and grinding process which uses at least three inductive sensors disposed around the periphery of the ball mill. A gradient signal which is produced by the sensors is used to define the dynamics of the process. Signals are also obtained for the mill content, and the degree of filling of the mill.
A more recent approach to the problem has been to incorporate a load cell in the foundations of a grinding mill. The cell monitors the mill mass and this, in conjunction with data on the power drawn by the mill motor, is used to control the rate at which ore is fed to the mill. This technique however does not lend itself to incorporation in existing mills which do not have the facility for inclusion of a load cell.